Using observations spanning a period of four years, a team of researchers from Italy found evidence of a large lake of salty water, buried 1.5 kilometres beneath Mars’ southern polar cap. That lake is at least 20 kilometres across, and seems to be a permanent feature.

The reason people are excited about this discovery is because on Earth, everywhere you find liquid water, you find life. NASA has long espoused a philosophy of “follow the water” in its program of astrobiological research – trying to answer the question “are we alone?”

Over the past two decades, we have seen mission after mission travel to Mars. Some, like Mars Express, are orbiters, whereas others (such as the incredible Spirit and Opportunity) are rovers. A unifying theme across those missions has been their attempts to see whether Mars once had the right conditions for life to exist and thrive.

Once life is established, it is amazingly hard to get rid of. Over millions of years, Mars cooled and its water became locked in permafrost. Its atmosphere thinned and it became the red planet we see today.

But maybe, just maybe, that life would have been able to follow the water – to move underground, where it might have found a niche, in a dark salty lake, buried beneath the ice of Mars’ southern polar cap.

That’s all well and good, but what next?

That’s all speculation, but it shows the kind of thought processes that have driven our ongoing exploration of Mars for the past couple of decades.

Now that we know for sure that there is a reservoir of liquid water just beneath the planet’s surface, astronomers around the globe will be thinking of ways to get down to that water to see what’s there.

That is easier said than done. Landing on Mars is challenging at the best of times, and the great majority of missions to date have landed within about 30° latitude of Mars’ equator. The two exceptions are the Viking 2 and Phoenix landers, both of which landed in Mars’ northern lowlands.

The locations on Mars’ surface visited by landers to date. It is far easier to land near Mars’ equator than its poles. NASA/JPL-Caltech

In addition, landing on Mars’ southern hemisphere is harder still. The north is the lowlands and the atmosphere there is markedly thicker, and the surface smoother (as befits, potentially, the floor of an ancient ocean).

To the south, you have less atmosphere to slow your descent and a rougher surface to make your landing harder.

But, while tricky, it is not impossible. And now we have a huge motivation to try.

It would not surprise me if, within a decade, we see missions being designed to visit Mars’ south pole and drill down to this great lake, to see what lurks within.